In general, mobile communication systems have been developed to provide communication services while guaranteeing user mobility. Thanks to rapid technical advancement, mobile communication systems are capable of providing not only voice communication services but also high-speed data communication services.
In recent years, the 3GPP (3rd Generation Partnership Project) has developed specifications for Long Term Evolution (LTE) as a next generation mobile communication system. The LTE system, also called Evolved Universal Terrestrial Radio Access Network (E-UTRAN), realizes high-speed packet based communication supporting a maximum data rate of about 100 Mbps exceeding existing data rates, aiming commercialization in 2010. To this end, various approaches have been considered, such as reducing the number of nodes on a communication path through simplification of the network architecture and bringing wireless protocols as close as possible to wireless channels.
Meanwhile, a resource to be allotted to a data service is determined according to the amount of data to be transmitted and a channel condition, unlike a voice service. Accordingly, in a wireless communication system, such as a mobile communication system, management is performed in such a way that a scheduler allocates transmission resources based on the amount of resources to be transmitted, a channel condition, and the amount of data. Such management is also performed in LTE systems, which are considered to be one of the next-generation mobile communication systems, and a scheduler located in a base station manages and allocates wireless transmission resources.
A User Equipment (UE) may connect to supportable systems, including not only an LTE communication system, but also other communication systems, such as a Universal Mobile Telecommunications System (UMTS), also called a UTRAN, in various ways such as inter-Radio Access Technologies (RAT) Handover (HO), cell-(re)selection, re-direction, and the like. Therefore, to stably support the connection, standardization and development have been conducted.